Accumulation of charges in polycarbonate due to positron irradiation

Positron annihilation lifetime spectroscopy (PALS) is a powerful technique for the study of free volume in polymers. The lifetime of ortho-positronium (o-Ps), a bound state of an electron and a positron, can be used to assess the cavity size, while the intensity can be used to characterize the number of cavities. In the past results have been published, which suffer from artifacts, whereby the drop in o-Ps intensity is not related to a decrease in the number of cavities. One of the possible artifacts is the build-up of an internal charge during long-term exposure to positron irradiation, which affects the o-Ps intensity. In this study we focus on the charging created in polycarbonate (PC) by positron radiation, which we investigated by the laser intensity modulation method (LIMM) and by charge-decay experiments, both isothermally and with TSD. From these measurements we conclude that: 1) the o-Ps intensity decrease during prolonged positron irradiation is due to the accumulation of a space charge; 2) in reverse, changes in the o-Ps intensity may be used to probe electric fields in dielectrics.